Generally, a sound of the hearing aid is hard in tone due to an echo effect by occluding the external ear canal and has a tendency of an impulsive sound to be emphasized. Therefore, there may be cases where it is not comfortable for wearers of hearing aids. Further, when sound pressures of wearers' personal voices are radiated in the occluded external ear canal via bone conduction, the personal voices may be unnaturally amplified so as to give an unnatural feeling that something is wrong. In conventionally employed methods for removing the uncomfortable feelings caused by occluding the external ear canal, the external ear canal is opened as much as possible, namely, like an open fitting, whereby the uncomfortable feeling may be reduced (for example, see patent references 1-3).    Patent reference 1: Japanese patent application publication No. 2001-508261    Patent reference 2: U.S. Pat. No. 6,275,596    Patent reference 3: U.S. Pat. No. 5,987,146